The present invention concerns generally a method and an apparatus for controlling automatic doors and, in particular, means for reducing the danger of catching a person in closing automatic doors for elevators.
Elevator safety regulations require safety devices which will prevent elevator users from being caught between closing elevator doors. The problem of providing such safety devices is typically solved by utilizing electomechanical closing force limiters which include a resilient element that is deflected when an impermissibly high force is exerted on the door. Upon deflection, the limiter actuates an electrical contact connected to a door control which initiates a reversing of the door driving motor to open the door.
A solution to the above described problem, in which an impermissible force influence on the door is detected without utilizing an electromechanical system, is shown in the U.S. Pat. No. 4,563,625. A voltage drop proportional to the motor current is interpreted as a torque value by means of a measuring resistor in the motor current circuit and is compared with an adjustable limit value. When the voltage exceeds the limit value, door stopping and reversing operations are initiated.
A substantial disadvantage of the above described solution is that the closing force must never exceed the maximum value which is permitted by the applicable safety regulations. This limitation reduces the accelerating force of the drive unnecessarily and eliminates the possibility of advantageously overloading an electric motor on a short term basis. Furthermore, in the case of a gradual change in the efficiency of the mechanical drive system, the consequence is a faulty response since the voltage drop no longer represents the actual torque and thus a false door fault is indicated.